Photographic characteristic of tabular silver halide grains (hereinafter referred to as "tabular grains") are described below.
1) Since the ratio of the surface area to the volume of a tabular grain is large, a large amount of a sensitizing dye can be adsorbed onto the surface of a grain, as a result, higher color sensitization sensitivity can be obtained;
2) When an emulsion which contains tabular grains are coated and dried, the grains are oriented in parallel with the surface of the support, therefore, the coated layer thickness can be made thin and the sharpness can be improved;
3) As tabular grains oriented in parallel with the support keep the shape and the orientation intact after development, the covering power of the developed silver is high. This characteristic, in particular in X-ray films, contributes to reduce the coating amount of silver required to obtain the same optical density;
4) Tabular grains oriented in parallel with the support exhibit less light scattering, therefore, an image having high definition can be obtained; and
5) As tabular grains are low sensitive to blue light, a yellow filter can be reduced or excluded from the emulsion when they are used in a green-sensitive layer or a red-sensitive layer.
In U.S. Pat. No. 4,439,520, a color photographic material whose sharpness, sensitivity and graininess are improved due to the use of tabular grains having a thickness of less than 0.3 .mu.m, a diameter of 0.6 .mu.m or more and an aspect ratio of 8 or more in at least one layer of a green-sensitive emulsion layer and a red-sensitive emulsion layer is disclosed. The aspect ratio used herein means the ratio of the diameter to the thickness of a tabular grain. The diameter of a tabular grain means the diameter of a circle having the same area as the projected area of a grain when an emulsion is observed using a microscope or an electron microscope. Further, the thickness of a tabular grain means the distance between two parallel main planes constituting the tabular grain.
In U.S. Pat. No. 4,693,964, a photographic element containing silver bromide or silver iodobromide tabular grains having an average grain size of from 0.4 to 0.55 .mu.m and an aspect ratio of 8 or more is disclosed. In U.S. Pat. No. 4,693,964, tabular grains having an average grain size of 0.5 .mu.m and a thickness of 0.04 .mu.m are disclosed in the working examples. In U.S. Pat. No. 4,672,027, a photographic element containing silver bromide or silver iodobromide tabular grains having an average grain size of from 0.22 to 0.55 .mu.m and an aspect ratio of 8 or more is disclosed. In U.S. Pat. No. 4,672,027, tabular grains having a thickness of 0.04 .mu.m are disclosed in the working examples.
In U.S. Pat. No. 5,250,403, a color photographic element containing tabular grains having {111} main planes and having an average grain size of 0.7 .mu.m or more and an average thickness of less than 0.07 .mu.m in minus blue layers (a green-sensitive layer and/or a red-sensitive layer) is disclosed. Tabular grains having an average thickness of less than 0.07 .mu.m are hereinafter referred to as "extremely thin" tabular grains. In U.S. Pat. No. 5,250,403, extremely thin tabular grains are attractive in view of the relationship between sensitivity and graininess, and it is advantageous to use extremely thin tabular grains in color photographic elements, in particular, in minus blue recording emulsion layers, in view of excellent sharpness of images.
In European Patent 362699, tabular grains having the ratio of the aspect ratio to the diameter of tabular grains of more than 0.7 are disclosed. In this European Patent, the preparation of tabular grains having a thickness of 0.04 .mu.m are disclosed in the working examples.
Thus, studies have been heretofore concentrated on the development of tabular grains having higher aspect ratio and thinner thickness to exhibit higher characteristics of tabular grains. On the other hand, the higher quality of photographs has been strongly required, therefore, the development of techniques capable of achieving higher sensitivity has been desired.
As described above, since tabular grains have high ratio of the surface area to the volume, a large amount of a sensitizing dye can be adsorbed onto the surface of a grain, as a result, higher color sensitization sensitivity can be obtained. It is thought that the light energy transmission efficiency to silver halide is improved in a sensitizing dye by increasing the light absorption rate by virtue of the adsorption of a large amount of a sensitizing dye and the higher sensitization of the spectral sensitivity can be achieved.
However, there is limitation on the adsorption amount of a sensitizing dye onto the surface of silver halide grains and it is difficult to adsorb a sensitizing dye of the amount more than the saturation adsorption of a single layer. Accordingly, in the present situation, the absorption rate of the incident light quantum of each silver halide grain in the spectral sensitization region is still extremely low even when tabular grains are used.
Methods suggested for resolving these drawbacks are shown below.
P. B. Gilman, Jr. et al. made a cationic dye adsorb on the first layer and an anionic dye on the second layer using electrostatic force as disclosed in Photographic Science and Engineering, Vol. 20, No. 3, page 97 (1976).
G. B. Bird et al. made a plurality of dyes multilayer-adsorb on silver halide and sensitization was effected by virtue of Forster type excitation energy transfer as disclosed in U.S. Pat. No. 3,622,316.
Sugimoto et al. performed spectral sensitization by energy transfer from a luminescent dye as disclosed in JP-A-63-138341 and JP-A-64-84244 (the term "JP-A" as use herein means an "unexamined published Japanese patent application").
R. Steiger et al. tried spectral sensitization by energy transfer from a gelatin-substituted cyanine dye as disclosed in Photographic Science and Engineering, Vol. 27, No. 2, page 59 (1983).
Ikekawa et al. performed spectral sensitization by energy transfer from a cyclodextrin-substituted dye as disclosed in JP-A-61-251842.
So-called connecting dyes respectively having two chromophores which are not conjugated separately and connected by a covalent bond are disclosed in U.S. Pat. Nos. 2,393,351, 2,425,772, 2,518,732, 2,521,944, 2,592,196 and European Patent 565083. However, these dyes were not dyes aiming at the improvement of light absorption rate. As the dyes aiming at the improvement of light absorption rate actively, G. B. Bird, A. L. Borror et al. made connecting type sensitizing dye molecules having a plurality of cyanine chromophores adsorb onto silver halide to heighten the light absorption rate and contrived sensitization by the contribution of energy transfer as disclosed in U.S. Pat. Nos. 3,622,317 and 3,976,493. Ukai, Okazaki and Sugimoto proposed in JP-A-64-91134 to connect at least one substantially non-adsorptive cyanine, merocyanine or hemicyanine dye containing at least two sulfo groups and/or carboxyl groups to a spectral sensitizing dye adsorbable onto silver halide.
L. C. Vishwakarma disclosed in JP-A-6-57235 a method of synthesizing a connecting dye by a dehydration condensation reaction of two dyes. Further, L. C. Vishwakarma showed in JP-A-6-27578 that a connecting dye comprising monomethine cyanine and pentamethine oxonol had red-sensitivity, but in this case spectral sensitization due to Forster type excitation energy transfer among dyes was not effected because the luminescence of the oxonol dye did not overlap with the absorption of the cyanine dye. Therefore, higher sensitization by the light converging function of the connected oxonol cannot be obtained.
M. R. Roberts et al. suggested spectral sensitization by a cyanine dye polymer in U.S. Pat. No. 4,950,587.
As described above, numerous examinations have been conducted heretofore for the improvement of light absorption rate, but none of them was satisfactory in higher sensitization effect and there remained such problems as the increase of intrinsic desensitization and development inhibition.
From the above reasons, techniques of spectral sensitization to improve the light absorption rate of silver halide to ensure higher sensitization have been demanded.